- Spiral galaxy
A spiral galaxy is a
galaxybelonging to one of the three main classes of galaxy originally described by Edwin Hubblein his 1936 work “The Realm of the Nebulae” [cite book |last=Hubble |first=E. P. |authorlink=Edwin Hubble |title=The Realm of the Nebulae |year=1936 |publisher=Yale University Press |location=New Haven |isbn=36018182] and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk of stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.
Spiral galaxies are named for the (usually two-armed)
spiralstructures that extend from the bulge into the disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot OB stars that inhabit them. Roughly half of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. Our own Milky Wayhas long been believed to be a barred spiral, although the bar itself is difficult to observe from our position within the Galactic disk. The most convincing evidence for its existence comes from a recent [http://www.astro.wisc.edu/sirtf/ survey] , performed by the Spitzer Space Telescope, of stars in the Galactic center. [cite journal | last=Benjamin | first=R. A. et al | year=2005 | month=September | title=First GLIMPSE Results on the Stellar Structure of the Galaxy. | journal=The Astrophysical JournalLetters | volume=630 | issue=2 | pages=L149–L152 | url=http://www.journals.uchicago.edu/doi/full/10.1086/491785 | accessdate= 2007-09-21 | doi=10.1086/491785]
Together with irregulars, spiral galaxies make up approximately 70% of galaxies in the local
Universe. [cite journal |author=Loveday, J. |year=1996 |month=February |title=The APM Bright Galaxy Catalogue. |journal=Monthly Notices of the Royal Astronomical Society |volume=278 |issue=4 |pages=1025–1048 |url=http://articles.adsabs.harvard.edu/full/1996MNRAS.278.1025L |accessdate= 2007-09-15] They are mostly found in low-density regions and are rare in the centers of galaxy clusters. [cite journal |author=Dressler, A. |year=1980 |month=March |title=Galaxy morphology in rich clusters - Implications for the formation and evolution of galaxies. |journal=The Astrophysical Journal |volume=236 |pages=351–365 |url=http://articles.adsabs.harvard.edu/full/1980ApJ...236..351D accessdate= 2007-09-15 |doi=10.1086/157753]
Spiral galaxies consist of several distinct components:
* A flat, rotating disc of (mainly young)
starsand interstellar matter
* A central stellar bulge of mainly older stars, which resembles an
* A near-spherical halo of stars, including many in
supermassive black holeat the very center of the central bulgeThe relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.
Spiral arms are regions of
starsthat extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sa and SBa galaxies, for instance, have tightly wrapped arms, whereas Sc and SBc galaxies have very "loose" arms (with reference to the Hubble sequence). Either way, spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable.
A bulge is a huge, tightly packed group of
stars. The term commonly refers to the central group of stars found in most spiral galaxies.
Using the Hubble classification, the bulge of Sa galaxies is usually composed of population II stars, that is old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are a great deal smaller, and are composed of young, blue, Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity), and others simply appear as higher density centers of disks, with properties similar to disk galaxies.
Many bulges are thought to host a
supermassive black holeat their center. Such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A*is believed to be a supermassive black hole. There is a tight correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge, the M-sigma relation.
The bulk of the stars in a spiral galaxy are located either close to a single plane (the
Galactic plane) in more or less conventional circular orbits around the center of the galaxy (the galactic centre), or in a spheroidal galactic bulge around the galactic core.
However, some stars inhabit a spheroidal halo or galactic spheroid. The orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the
Sagittarius Dwarf Elliptical Galaxyis in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.
Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of
Population II, much older and with much lower metallicitythan their Population Icousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.
The motion of halo stars does bring them through the disc on occasion, and a number of small
red dwarfstars close to the Sunare thought to belong to the galactic halo, for example Kapteyn's Starand Groombridge 1830. Due to their irregular movement around the centre of the galaxy—if they do so at all—these stars often display unusually high proper motion.
Origin of the spiral structure
The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was
Bertil Lindbladin 1925. He realised that the idea of stars arranged permanently in a spiral shape was untenable due to the "winding dilemma". Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy, a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Or, the stars on the outermost edge of the galaxy would have to move faster than those near the center, as the galaxy rotates. Neither behaviour is observed.
There are two leading hypotheses or models for the spiral structures of galaxies:
* Star formation caused by density waves in the
galactic diskof the galaxy.
SSPSF model- Star formation caused by shock waves in the interstellar medium.
These different hypothesis do not have to be mutually-exclusive, as they may explain different types of spiral arms.
Density waves model
Bertil Lindbladproposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy’s stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.
This idea was developed into
density wave theoryby C. C. Linand Frank Shuin 1964.cite journal |last=Lin |first=C. C. |authorlink= |coauthors=Shu, F. H. |year=1964 |month= August|title=On the spiral structure of disk galaxies. |journal=The Astrophysical Journal |volume=140 |pages=646–655 |url=http://articles.adsabs.harvard.edu/full/1964ApJ...140..646L |accessdate=2007-09-26|doi=10.1086/147955] They suggested that the spiral arms were manifestations of spiral density waves, attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars.
Historical theory of Lin and Shu
The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964.
* They suggested that the spiral arms were manifestations of spiral density waves.
* They assumed that the stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic centre. This is illustrated in the diagram. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.Fact|date=February 2008
tar formation caused by density waves
The following hypotheses exist for star formation caused by density waves:
*As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the
Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
*As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
*As clouds get swept up by the spiral arms, they collide with one another and drive
shock wavesthrough the gas, which in turn causes the gas to collapse and form stars.
hock waves model
Alternative hypotheses that have been proposed involve waves of star formation moving about the galaxy, also called the stochastic self-propagating star formation model or
SSPSF model. This model proposes that star formationpropagates via the action of shock waves produced by stellar winds and supernovae that compose the interstellar medium.
These different hypothesis for the spiral arms do not have to be mutually-exclusive, as they may explain different types of spiral arms.
More younger stars in spiral arms
The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the (darker) background stellar distribution behind the waves, hence making the waves visible.
While stars, therefore, do not remain forever in the position that we now see them in, they also do not follow the arms. The arms simply appear to pass through the stars as the stars travel in their orbits.
Alignment of spin axis with cosmic voids
Recent results suggest that the orientation of the spin axis of spiral galaxies is not a chance result, but instead they are preferentially aligned along the surface of cosmic voids. [cite journal | last = Trujillo | first=I. | coauthors=Carretero, C.; Patiri, S.G. | title=Detection of the Effect of Cosmological Large-Scale Structure on the Orientation of Galaxies | journal=The Astrophysical Journal | year=2006 | volume=640 | issue=2 | pages=L111–L114 | url=http://adsabs.harvard.edu/abs/2005astro.ph.11680T | doi=10.1086/503548 ] That is, spiral galaxies tend to be oriented at a high angle of inclination relative to the large-scale structure of the surroundings. They have been described as lining up like "beads on a string," with their axis of rotation following the filaments around the edges of the voids. [cite web | last = Alder | first = Robert | year = 2006 | url = http://www.astronomy.com/asy/default.aspx?c=a&id=4215 | title = Galaxies like necklace beads | publisher = Astronomy magazine | accessdate = 2006-08-10 ]
“Spiral nebula” is an old term for a spiral galaxy. Until the early 20th century, most astronomers believed that objects like the
Whirlpool Galaxywere just one more form of nebulathat were within our own Milky Waygalaxy. The idea that they might instead be other galaxies, independent of the Milky Way, was the subject of The Great Debateof 1920, between Heber Curtisand Harvard-based Harlow Shapley. In 1926, Edwin Hubble[cite journal |author=Hubble, E. P. |authorlink=Edwin Hubble |year=1926 |month=May |title=A spiral nebula as a stellar system: Messier 33. |journal=The Astrophysical Journal |volume=63 |pages=236–274 |url=http://articles.adsabs.harvard.edu/full/1926ApJ....63..236H |accessdate= 2007-09-21 |doi=10.1086/142976] observed Cepheid variablesin several spiral nebulae, including the Andromeda Galaxy, proving that they are, in fact, entire galaxies outside our own. The term “spiral nebula” has since fallen into disuse.
Galaxy color-magnitude diagram
Galaxy morphological classification
Barred spiral galaxy
Dwarf elliptical galaxy
Dwarf spheroidal galaxy
Intermediate spiral galaxy
Unbarred spiral galaxy
Galactic coordinate system
Galaxy formation and evolution
Groups and clusters of galaxies
List of galaxies
List of nearest galaxies
Timeline of galaxies, clusters of galaxies, and large scale structure
* [http://www.datasync.com/~rsf1/avm-bib.htm On Spiral Nebulae, Adriaan van Maanen, et al] This article deals with the view that the nebulae are associated with the Milky Way.
* [http://www.seds.org/messier/spir.html Spiral Galaxies @ SEDS Messier pages]
* [http://spiralzoom.com/Science/spiralgalaxies/SpiralGalaxies.html "SpiralZoom.com"] , an educational website about Spiral Galaxies and other spiral formations found in nature. For high school & general audience.
* [http://burro.cwru.edu/Academics/Astr222/Galaxies/Spiral/spiral.html Spiral Structure explained]
Wikimedia Foundation. 2010.